U.S. patent application number 12/704812 was filed with the patent office on 2011-08-18 for investment casting of induction motor rotors.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Scott W. Biederman, Mark A. Osborne, Thomas A. Perry, Michael J. Walker.
Application Number | 20110198964 12/704812 |
Document ID | / |
Family ID | 44369169 |
Filed Date | 2011-08-18 |
United States Patent
Application |
20110198964 |
Kind Code |
A1 |
Biederman; Scott W. ; et
al. |
August 18, 2011 |
INVESTMENT CASTING OF INDUCTION MOTOR ROTORS
Abstract
A procedure for casting a shorted structure around a plurality
of induction motor rotors is described. The method comprises
forming a wax representation of the shorted structure around a
lamination stack; mounting a plurality of such lamination stacks in
a mounting fixture and attaching a suitable gating and runner
system; forming an investment by coating the structure with
refractory followed by melting out the wax; casting molten metal
into the investment while it is rotating and aligning the mold to
allow the centrifugal force generated to promote mold filling; and,
continuing to rotate the investment until solidification is
substantially complete.
Inventors: |
Biederman; Scott W.; (New
Boston, NH) ; Osborne; Mark A.; (Grand Blanc, MI)
; Walker; Michael J.; (Windsor, CA) ; Perry;
Thomas A.; (Bruce Township, MI) |
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
44369169 |
Appl. No.: |
12/704812 |
Filed: |
February 12, 2010 |
Current U.S.
Class: |
310/211 ;
29/598 |
Current CPC
Class: |
Y10T 29/49012 20150115;
Y10T 29/49009 20150115; B22C 9/04 20130101; B22D 13/06 20130101;
H02K 15/0012 20130101 |
Class at
Publication: |
310/211 ;
29/598 |
International
Class: |
H02K 17/16 20060101
H02K017/16; H02K 15/02 20060101 H02K015/02 |
Claims
1. A method for forming a cast shorted structure on the rotor of an
induction motor, the rotor having a rotor rotation axis and
comprising a plurality of generally planar ferrous laminations
arranged to form a lamination stack, the lamination stack being
generally cylindrical and having two ends and a surface; the
shorted structure comprising two end rings coaxial with the rotor
rotation axis, one of each end rings abutting each end of the
lamination stack and a plurality of conductor bars positioned
proximate to the surface of the lamination stack and connected on
their ends to the end rings, the method comprising; applying a wax
pattern of the shorted structure to the rotor; attaching at least
one wax pattern of a sprue and runner structure to the wax shorted
structure pattern to form an investment pattern; coating the
investment pattern with a refractory coating to form an investment;
removing the wax from the investment; positioning the investment
with the sprue positioned on and aligned with a rotation axis and
with the rotor rotation axis inclined so as to be generally
perpendicular to the rotation axis; filling the investment with
molten casting metal through the sprue while rotating the
investment about the rotation axis to urge outwardly radial flow of
the casting metal from the sprue in a direction generally aligned
with the conductor bar, the molten metal first entering the runner,
then a first mold end ring region, then the mold runner region
before entering a second mold end ring region; allowing the molten
metal to solidify while the mold shell is continuing to rotate; and
removing the gates and runners.
2. The method of claim 1 further comprising preheating the
investment to a temperature substantially equal to the melting
point of the casting metal.
3. The method of claim 2 further comprising supporting the
investment in sand.
4. The method of claim 1 where the casting metal comprises one of
the group consisting of substantially pure electrical grade copper,
a copper alloy, substantially pure electrical grade aluminum, and
an aluminum alloy.
5. A method for forming a cast shorted structure on each of a
plurality of induction motor rotors, each rotor comprising a
plurality of generally planar ferrous laminations arranged and
aligned to form a lamination stack, the lamination stack being
generally cylindrical and having two ends and a surface; the cast
shorted structure comprising two end rings coaxial with the rotor
rotation axis, one of each end rings abutting each end of the
lamination stack and a plurality of conductor bars positioned
proximate to the surface of the lamination stack and connected on
their ends to the end rings, the method comprising; fabricating a
mold of the cast shorted structure, the mold incorporating at least
the rotor laminations, and comprising at least a runner for ingress
of molten metal, the runner being connected to the mold at the
location of one of the rotor end rings; forming a substantially
balanced mold assembly by assembling a plurality of molds arranged
in substantial opposition to one another with their runners
connected to a common central sprue, the sprue having a centerline
with each of the molds being positioned on a common plane and each
of the molds oriented with their conductor bars substantially
perpendicular to the sprue; the mold assembly having a rotation
axis substantially aligned and coincident with the centerline of
the sprue; filling the mold assembly with molten casting metal
introduced through the sprue while rotating the mold assembly about
the rotation axis to urge outwardly radial flow of the casting
metal from the sprue, through the runner and a first end ring mold
region, then along the conductor bar mold regions to the second end
ring mold region; and allowing the molten metal to solidify while
the mold assembly continues to rotate.
6. The method of claim 5 further comprising preheating the mold
assembly to a temperature substantially equal to the melting point
of the casting metal.
7. The method of claim 5 further comprising attaching at least a
second mold assembly comprising a second sprue to a first mold
assembly comprising a first sprue, the mold assemblies being so
positioned and aligned that their sprues may be attached to create
a continuous path for the molten metal and enable filling of all
mold assemblies during a single pour.
8. The method of claim 5 where the casting metal comprises one of
the group consisting of substantially pure electrical grade copper,
a copper alloy, substantially pure electrical grade aluminum, and
an aluminum alloy.
9. The method of claim 7, further comprising a first fixture for
positioning and supporting the molds comprising the first mold
assembly and a second fixture for positioning and supporting the
molds comprising the second mold assembly, the first and second
fixtures being cooperatively adapted for releaseable attachment to
one another.
10. The fixture of claim 9 where the first and second fixtures
comprise at least one element adapted for alignment of the first
fixture and the second fixture.
11. The elements of claim 10 where the element is a hollow rib of
uniform cross-section comprising an attachment at a first end, the
attachment being rigidly attached to the rib, the attachment being
of progressively reducing cross-section so that as the attachment
of a first rib is progressively inserted into the opening at a
second end of a second rib the attachment will guidably engage the
opening of the second hollow rib to align the first and second ribs
when full insertion is achieved.
12. The ribs of claim 11 further comprising releasable locking
features on the attachment and the second ends of the ribs such
that full insertion of the attachment of the first rib into the
opening of the second rib will promote a mechanical interference
between the attachment and the second rib.
13. A cast rotor for induction motor fabricated by the method of
claim 1.
14. The cast rotor for an induction motor of claim 13 wherein the
shorted structure comprises a copper alloy.
15. The cast rotor for an induction motor of claim 13 wherein the
shorted structure comprises an aluminum alloy.
Description
TECHNICAL FIELD
[0001] This application relates to processes for forming shorted
structures comprising conductor bars and end rings on laminated
cores of rotors for induction motors. More specifically, this
disclosure relates to investment casting of shorted structures on
rotor cores using rotating mold assemblies by which pairs of rotors
may be produced at the same time.
BACKGROUND OF THE INVENTION
[0002] One candidate electric motor type for driving wheels of
electric and hybrid vehicles is the induction motor. Induction
motors, of course, may be designed in many different sizes and
shapes for delivering rotational power.
[0003] A typical induction motor has a stationary annular
wire-wound outer member of designed diameter and length called a
stator. Often a three-phase alternating current is delivered to
electrical leads of the stator so as to produce a magnetic field
that rotates around the stator ring. A cylindrical rotor member
carried on the rotating power shaft for the motor is placed closely
spaced within the inner cylindrical cavity of the stator. The rotor
has an inner cylindrical core of flat round steel plates, coated
with electrically insulating material, and stacked as laminations
with their circumferences aligned to form the cylindrical core so
that it has a length complementary to that of the stator. This
cylindrical core does not conduct electricity but it displays high
electromagnetic permittivity.
[0004] Each laminated disk of the rotor core may be shaped with
circumferential indentations, or the like, to carry several (e.g.,
20-40) uniformly spaced, equal length, copper or aluminum
electrical conductor bars extending from one end of the rotor core
to the other. The spaced conductor bars may be uniformly slightly
inclined to the cylindrical axis of the rotor core and the ends of
each bar are connected to copper or aluminum end rings located on
the rotor ends and co-axial with the rotor axis. This one-piece,
cage-like structure of spaced and inclined conductor bars with end
rings, carried on the laminated rotor core, is highly electrically
conductive and termed a "shorted structure."
[0005] Because only a small clearance is maintained between stator
and rotor, the rotating magnetic field of the stator enters the
rotor, inducing a current in the embedded conductors. In turn, the
conductor current produces its own magnetic field which is repelled
by the stator magnetic field and causes the rotor to rotate.
Inclination of the conductor bars with respect to the rotational
axis of the rotor cooperates with the rotation of the magnetic
field produced by the stator and permits a more uniform production
of torque by the induction motor.
[0006] The shorted structure may be fabricated by assembly and
joining of its individual components, the conductor bars and end
rings. An alternative approach, which promised a shorter
manufacturing time, has been to overcast the conductor bars and end
rings as a complete structure on the lamination stack using die
casting. However, rotors manufactured using the die casting
approach have exhibited problems with excessive porosity and lower
than optimum shorted structure (electrical) conductivity which has
reduced process yield.
[0007] Thus there is need for a process for rapidly fabricating
induction motor rotors and particularly the shorted structure of
such rotors.
SUMMARY OF THE INVENTION
[0008] This invention provides a method for casting the shorted
conductor bar structure of an induction motor rotor onto a rotor
lamination stack in a manner which enables consistent quality and
high production rates. The shorted structure typically comprises
many equal-length conductor bars and two end rings. Conductor bars,
oriented to be aligned generally at an acute angle with the
rotational axis of the rotor, extend the length of the rotor
lamination stack and are equally spaced around the circumference of
the rotor lamination stack. The conductor bars terminate in the end
rings, one of which is positioned at each extremity of the
lamination stack. The conductor bars are contained within and
thereby mechanically restrained by the lamination stack while being
generally positioned near the circumference of the rotor stack.
[0009] The method applies investment (or "Lost Wax") casting
process practices to a mold assembly comprising at least one mold
suited for casting of a unitary shorted structure on a
complementary laminated plate stack. The shorted structure
comprises a first end ring attached to one end of a number of
conductor bars and a second end ring attached to the other end of
the conductor bars. Each mold will be constructed to permit the
entry and flow of molten metal in the direction from one end ring
of the conductor bars to their other end ring. The mold body is
rotated about a rotation axis in a circular path with the laminated
plate stack axis (the rotor axis) aligned with a radius of the
circular path and molten metal is introduced at the rotation axis.
Thus, the resulting centrifugal forces are suitably directed to
efficiently urge the molten casting alloy into the mold along the
rotation axis of the rotor to enhance feeding of any metallurgical
shrinkage that may develop. Molten metal first enters the mold at a
mold cavity corresponding to an end ring, then progresses along
mold channels corresponding to the conductor bars and finally fills
the mold cavity corresponding to the opposing end ring. Thus the
mold orientation promotes metal flow in a direction substantially
corresponding to the conductor bar orientation.
[0010] It is apparent pairs of diametrically opposing molds for the
rotor structures may be rotated in combinations with the metal fed
from the center of rotation of the opposing rotor mold assemblies.
Thus, this casting process may be conducted to enable simultaneous
casting of conductor bars and end rings for a plurality of rotors
to efficiently enable higher volume production. Hence, the
orientation of each of the plurality of rotors will be suitable for
constructive utilization of the centrifugal force by all rotors.
Thus, some number of rotor molds may be radially disposed about the
rotation axis. To minimize imbalance during rotation, rotor molds
may be positioned in the mold in generally symmetrical
configurations, usually with pairs of molds arranged in opposition
and disposed at generally equal distances from the rotation axis.
Such configuration will result in an assemblage of laterally-spaced
rotor molds all of which are located at a common height and thereby
form a mold layer. Yet higher production volumes may be obtained by
suitably stacking a plurality of such mold layers to enable casting
additional rotors during a single pouring operation of the molten
metal at the centers of rotation of the several molds.
[0011] The mold making process comprises molding a wax form or
pattern corresponding to the geometry of the desired shorted
structure around a rotor lamination stack or stacked individual
laminations. Then a ceramic mold, an investment, is developed by
application of ceramic particles to a form comprising a plurality
of rotors and their associated wax pattern of the shorted
structure, individually attached to a wax runner pattern and with
each runner assembled to a common wax sprue pattern. The investment
is heated to a temperature sufficient to melt the wax which is
substantially drained from the investment. Further heating, to a
much higher temperature, combusts the remaining wax and preheats
the investment so that its temperature more closely matches the
temperature of the casting metal. The investment is then oriented
appropriately to optimize mold filling as described above,
supported in compacted sand and fed with liquid metal while being
rotated about an axis generally corresponding to the centerline of
the common sprue. Although other configurations may be employed, it
is preferred that the conductor bars be aligned generally parallel
to the resultant centrifugal force and that the molten metal enters
the mold at one end ring, thereafter progressing along the
conductor bars and subsequently filling the end ring opposite the
one by which it entered. Rotation is maintained until
solidification is substantially complete.
[0012] The above features and advantages and other features and
advantages of the present invention are readily apparent from the
following detailed description of the best modes for carrying out
the invention when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A shows an exemplary rotor design and construction
suitable for use in an induction motor.
[0014] FIG. 1B shows a partially exploded view of the exemplary
induction rotor of FIG. 1A better illustrating the
interrelationship between the lamination stack and the conductor
bars and end rings, which in combination, comprise a shorted
structure. Also shown are two planes of partial section, ABCD and
EFGH which, when combined produce the combination section of FIG.
3. The two end rings shown in exploded view at the left end of FIG.
1B are illustrated without their conductor bars to better show
their retaining slots for conductor bars.
[0015] FIG. 1C shows a fragmentary plan view of a lamination as
shown in FIG. 1B to better illustrate some of its features.
[0016] FIG. 2 shows, in partial cutaway, a second rotor design,
suitable for the practice of this invention.
[0017] FIG. 3 shows, in schematic fashion, a combination sectional
view of a mold suitable for application of a wax overcoat to a
lamination stack. The view combines two sections, each taken along
the inclined conductor bar mold cavities. Thus the combination
section combines the section taken along plane ABCD of FIG. 1B and
the section taken along plane EFGH of FIG. 1B thereby showing the
conductor bar as continuous in both segments. For clarity the
sectioning planes of FIG. 1B are identified.
[0018] FIG. 4 shows an assembly of six rotor shorted structure wax
patterns, for forming the rotor design of FIG. 2, positioned in a
circle as three sets of opposing mold pairs on one planar layer of
a centrifugal casting fixture with wax runner patterns attached to
a common wax sprue pattern at the center of the axis of rotation of
the mold assembly.
[0019] FIG. 5 shows a centrifugal casting wax pattern suitable for
casting a shorted structure on a plurality rotors. The pattern has
two stacked layers, each layer being as shown in FIG. 4, and
illustrating the rotational axis employed during casting. A common
wax sprue pattern is used for all layers.
[0020] FIG. 6 shows an embodiment of a feature of the centrifugal
casting fixture to better restrain relative motion of the stacked
layers of FIG. 5.
[0021] FIGS. 7A, B and C show a second embodiment of a feature of
the centrifugal casting fixture to better restrain relative motion
of the stacked layers of FIG. 5 and illustrate its mode of
operation.
[0022] FIG. 8 shows a portion of the six rotor shorted structure
wax patterns shown in FIG. 4 after further processing to coat the
pattern with ceramic and remove the wax to form an investment
suitable for receiving molten metal.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0023] Induction motors operate through the repulsive interaction
of a rotating electrically-generated magnetic field in a stator
with an induced magnetic field arising from the induced current in
an arrangement of conductors positioned on the rotor. Induction
motors enjoy wide application and are available in a range of
configurations depending primarily on their electrical rating but
influenced also by packaging constraints. Thus many variants of the
motor elements exist. In particular, the rotors may exhibit
pronounced differences in length, diameter etc.
[0024] In common with other motors, particularly large motors
suitable for automotive application, the magnetic forces are
substantial and require that any conductors be restrained and
securely anchored. Thus the rotor conductors are typically not
positioned on the surface of the rotor but are instead embedded,
partially or completely, within the rotor so that they may be well
supported by the rotor structure.
[0025] A typical rotor 10 is illustrated in FIG. 1A showing a
lamination stack 11 (a stack of bonded disks or sheets of like
shape) surrounding a supporting shaft 12 with splines 13 on one
end. The lamination stack is surrounded by shorted structure 20
comprising conductor bars 14 and end rings 16 and 17, the end rings
16 and 17 and conductor bars 14 being connected together to form an
electrically-conductive cage, shorted structure 20, around the
lamination stack 11. In this example the conductor bars 14 are
shown as inclined to the axis of rotation of the rotor 10, a
relatively common configuration adopted to minimize motor speed
variations or torque ripple.
[0026] The lamination stack 11 is fabricated as a laminated
assemblage of generally annular shaped plates or disks cut or
stamped from rolled sheet, usually by a blanking process using
matched dies mounted in a sheet metal press. Less frequently laser
cutting or electrical discharge machining may be employed. The
individual disks are then suitably aligned and stacked atop one
another, usually separated by an interposed electrically insulating
layer or coating, and permanently attached to one another. Most
often the laminations are fully formed as-separated and assembled
by carefully positioning one lamination atop another in prescribed
orientation. Less-commonly the desired external features are
imparted by a separate machining operation conducted on the
lamination stack after their assembly.
[0027] The laminations are magnetically `soft`, that is readily
magnetized, and typically prepared from electrical steel with a
chemistry largely comprising iron with up to 6 percent silicon by
weight and less than 0.005 percent by weight carbon. A
commonly-used composition is iron with 3 weight percent
silicon.
[0028] Additional details of rotor 10 may be noted by consideration
of partially-exploded view FIG. 1B. This partially exploded view
shows two individual laminations 18 separated from lamination stack
11 and illustrates the form of the conductor bar retaining slots
19. The conductor bar retaining slots 19, as better seen in FIG.
1C, are generally shaped like the letter "V" but have a partially
closed opening at the rotor periphery 21 to restrain the conductor
bar against expulsion due to the high magnetic forces. FIG. 1C also
shows a feature 15 to aid in angularly locating a lamination to the
shaft 12 as will be discussed in greater detail later.
[0029] The shorted structure may be fabricated as an assembly.
However a more promising approach is to cast the shorted structure
as a single piece over the lamination stack 11. Such an approach is
challenged by the thermal mass of the lamination stack which will
tend to rapidly extract heat from the inflowing molten metal and
may choke off the flow of molten liquid prematurely causing flow
passages to freeze before the mold fills completely. Die casting,
which may employ a water cooled mold and uses mechanical assistance
to rapidly charge the molten liquid to the mold, has been used but
has generally failed to consistently generate the desired quality
or to deliver the expected productivity enhancement required by
hybrid traction motors.
[0030] The subject invention employs a one piece ceramic mold or
investment formed using the lost wax process. The mold is then
rotated before being charged with molten metal. Rotation is
maintained during pouring and continues until solidification
occurs. Rotation induces and generates a centrifugal force which,
in combination with appropriate mold positioning will be effective
in urging the molten metal into the mold and promoting mold fill
before the conductor bars, sprue and/or runner structure freezes
and prohibits further metal addition. It is preferred that the
direction of rotation be such as to generate a centrifugal force
which acts in a direction parallel to the conductor bars.
[0031] As is well known, because of shrinkage and contraction, the
volume of a casting is usually less than the volume of the mold
into which it is cast. Thus, suitable adjustment to the mold
dimensions, usually described as a pattern-maker's allowance, is
made to ensure the finished casting dimensions. These
considerations apply to the process under discussion. Thus, where
reference is made to a wax pattern it will be appreciated that the
general geometry of the cast feature and the pattern will be
substantially identical but that the dimensions of the wax and cast
features will differ.
[0032] FIG. 2 shows a rotor 10' of a second design with a
lamination stack 11' comprising laminations 18' and shorted
structure 20' comprising conductor bars 14' and end rings 16', 17'.
Rotor 10' is likewise representative of those suitable for practice
of this invention. It will be appreciated that the details of rotor
10' differ from those of rotor 10 with respect to at least length,
external diameter, internal diameter and conductor bar placement.
Such design variances are commonly encountered and are not
prejudicial to the practice of this invention which is intended for
broad application to induction motor rotor variants in common
use.
[0033] A point of difference between the rotor design of FIGS. 1
and 2 is that in FIG. 2 the conductor bars 14' are fully surrounded
by the lamination stack 11'. Inasmuch as the conductor bar opening
will generally be formed during a single press stroke such a
feature may be readily accommodated. This design ensures that if
the rotor is subjected any machining or grinding processes for
balance or concentricity or to achieve rotor-stator clearance
tolerances, the current-carrying capability of the conductor bar
will not be compromised.
[0034] FIG. 3 shows, a composite section, obtained by combining
sections like those shown at ABCD and EFGH in FIG. 1B, of a
lamination stack 11 positioned in a split mold 50 suitable for
casting wax in locations corresponding to the desired locations of
conductor bars 14 and end rings 16 and 17 shown in FIG. 1B. The
planes of section ABCD and EFGH as shown in FIG. 1B are chosen to
enhance the clarity of the figure and specifically to convey that
the conductor bar openings 114, are continuous. It will be
appreciated that conductor bar openings 114 of FIG. 3 are intended
to receive cast, electrically conducting material and thereby form
conductor bars 14 of FIGS. 1A and 1B with a section as best shown
at 19 in FIG. 1C. Similarly end ring openings 116 and 117 of FIG. 3
when filled with cast electrically conducting material will form
end rings 16 and 17 respectively of FIGS. 1A and 1B. Thus the
planes of section are inclined to the rotational axis of the rotor
and sectioning planes ABCD and EFGH are oppositely inclined to the
rotational axis. The individual sections corresponding to
sectioning planes ABCD and EFGH have been combined in the composite
section of FIG. 3. It will be appreciated that FIG. 3 will show a
true section for a rotor geometry where conductor bars are not
inclined to the axis of rotation but instead are parallel to the
rotation axis.
[0035] The wax-casting mold 50 is intended to be reusable and will
generally be fabricated of metal for durability. Since the low
melting point of wax does not mandate use of more heat resistant
materials, aluminum alloy is a suitable mold material and offers
easy machining. It will be appreciated that operation of the mold
will require that it be mounted in a press or similar device and
require additional features such as a guide pins, mounting plates
etc. which have been omitted for simplicity.
[0036] The mold comprises a first mold section 58 including a core
feature 59 and a second mold section 60 separated along a parting
line XX. The mold incorporates provision for injection of molten
wax through runner 52 and has vents 56. The cylindrical periphery
21 of lamination stack 11 is fitted tightly to the cylindrical
walls 66 of second mold section 60 to effectively bar deposit of
wax on the outer periphery of lamination stack 11. Further, the
close fit between the laminations and the mold section facilitates
aligning the laminations. A similarly close fit is desired between
the inner bore of the laminations and the outer surfaces of core
59. Introduction of complementary features on the inner bore of the
laminations and the outer surfaces of core 59 may also be used to
facilitate alignment of stacked laminations. For example the
inclination of conductor rods 14 as shown in FIG. 1A would be
readily achieved by inclining a protuberance (not shown) on core 59
complementary to a slot or recess on the bore of the lamination
such as is shown at 15 in FIG. 1C, or vice versa. It may also be
noted that the cast conductor rods will act to mechanically secure
the laminations to form the lamination stack so that the
laminations may be loaded into the mold, or more preferably, onto
the core individually, potentially facilitating stack assembly.
[0037] The outwardly-facing end lamination 70 of lamination stack
11 is sealingly spaced apart, such as by stops 72, from second mold
surface 68 to create annular opening 116. Similarly the
outwardly-facing end lamination 71 of lamination stack 11 is
sealingly spaced apart, such as by stops 74, from first mold
surface 76 to create annular opening 117. Thus molten or flowable
injection molding wax formulated from hydrocarbon wax, natural
ester wax, synthetic wax, natural and synthetic resins, organic
filler materials and water to achieve suitable characteristics as
is well known to those skilled in the art, may be introduced
through runner 52. As depicted in FIG. 3, the wax on entering the
mold will first fill the annular region 116 corresponding to a
first end ring, then flow along channels 114 corresponding to the
conductor bars before filling regions 117 on the opposing end
surface of the rotor to form the second end ring. Vents 56 will
enable venting of air initially present in the mold. Alternatively
the mold may be evacuated prior to introduction of wax and the
vents eliminated.
[0038] When the wax has solidified and hardened, mold segments 58
and 60 may be separated along split line XX by motion in a
direction indicated by double arrow YY. As depicted, the wax
over-molded lamination stack including the wax runner pattern
(designated 52' in FIGS. 4 and 5) may now be readily removed from
the mold, if necessary with the aid of an ejector pin (not shown),
again along direction YY and the wax sections corresponding to
vents 56, if present, removed.
[0039] It will be appreciated that FIG. 3 is exemplary and not
restrictive and that alternate mold designs incorporating different
wax fill geometries and mold segment geometries may be employed.
Such variants are fully comprehended by the invention. Further,
although not preferred, the wax features corresponding to the
conductor bars, end rings and runner, may also be built up by hand,
for example, by laying up shaped wax forms and attaching them
together by co-melting the contacting forms.
[0040] FIG. 4 shows a plurality of wax-overmolded lamination stacks
110' after being overmolded with wax in a mold such as shown in
FIG. 3. These overmolded lamination stacks are positioned on a
casting fixture 100 which includes a wax sprue pattern 80, attached
to all of the wax runner patterns 52' associated with each of the
wax-overmolded lamination stacks 110'. Attachment of wax runner
pattern 52' to wax sprue pattern 80 is accomplished by co-melting
and adjoining the abutting portions of each individual pattern.
Preferably a common wax is used for all patterns so that when the
wax cools and solidifies it will constitute a joint with the same
characteristics as the pattern features. Wax-overmolded lamination
stacks 110' are derivative of the rotor 10' shown in FIG. 2 and
comprise lamination stack 11', wax features 114' corresponding to
conductor bars 14', wax feature 116' corresponding to end ring 16'
and wax feature 117' corresponding to end ring 17'. These features
are shown and indicated on only one of the overmolded lamination
stacks of the figure but are common to all overmolded lamination
stacks.
[0041] The wax-overmolded lamination stacks are positioned in
opposition to facilitate balance and are individually supported on
a supporting feature 84 dimensioned to slidably engage the inner
diameter of wax-overmolded lamination stacks 110' with minimal
clearance. Supporting features 84 are themselves attached to a
supporting structure comprising a stacked array of annuli 82
supported and attached by a plurality of ribs 86. All wax runner
patterns 52' are attached to a common wax sprue pattern 80. The
eventual axis of rotation 81, corresponding to the centerline of
wax sprue pattern 80 is also shown.
[0042] The structure depicted for the fixture is illustrative only
and various modifications to the structure shown are comprehended
in this invention. Without limitation these may include: variations
in rotor support features 84; or variations in the number or
distribution of rotors accommodated provided the resulting assembly
is substantially balanced; or of the nature of the supporting
structure 82; or of its support members 86. For example: the rotor
and shaft assembly of FIG. 1 might be supported using an internally
splined hollow cylinder sized to slidably engage splines 13 on
shaft 12 of FIG. 1; the supporting structure might comprise more or
fewer annular features like that shown at 82 in FIG. 4 and the
features might be of greater or lesser diameter and/or of alternate
cross-section; and finally the support members shown as 86 in FIG.
4 might be modified in number, cross-section or incorporate
additional features for improved performance as illustrated in
FIGS. 6 and 7.
[0043] FIG. 5 illustrates a partial build-up of a casting fixture
300 comprised of two of the layers shown in FIG. 4, depicted as a
first layer 100 and a second layer 200. Ribs 86 are aligned and
serve to releasably join first layer 100 to second layer 200. For
convenience the rotors in each of the layers are depicted as
aligned but it may be advantageous to stagger the rotor positioning
in the different layers if such a configuration improves rotational
balance. As will become clearer from the discussion of the methods
of attaching the layers staggering the rotor orientation may
require that additional ribs 86 be provided. The additional ribs
may be positioned symmetrically in the substantially 120.degree.
sectors between the ribs depicted in FIGS. 4 and 5.
[0044] Any convenient attachment procedure may be followed. For
example as represented in FIG. 4, ribs 86 are hollow. Thus layer
alignment may be enabled by sliding a tight-fitting rod of
complementary shape through ribs 86 of each layer and tying layers
100 and 200 together with wire or other suitable material.
[0045] Alternatively, in a second embodiment, best illustrated at
FIG. 6, ribs 86' and 86'', respectively mounted on annular features
82' and 82'', are shown. Ribs 86', 86'' have been formed by the
incorporation of shaped plugs 87' and 87'' permanently attached to
one end of each of ribs 86' and 86'', for example by riveting or
other mechanical fastener or by welding or by interference fit or
other suitable means. Plug 87' extends beyond surface 92 of rib 86'
and is adapted for easy insertion into the open end of rib 86'',
for example by adoption of a tapered cross-section as shown. Thus,
as shown in FIG. 6, layer 200 may, provided ribs 86' and 86'' are
approximately aligned, be positioned atop layer 100, enabling plug
87' to guide and engage the opening of rib 86'' so that end surface
92 of rib 86' is brought into contact with surface 94 of rib 86''.
Thus layers 100 and 200 are locked together against rotation but
again would require tying together to restrain them from being
pulled apart.
[0046] In a yet further variant shown in FIG. 7A, tapered plug 89
attached to one end of rib 86' incorporates a recess 96 and rib
86'' incorporates a pin 99, complementary in shape to recess 96,
extending through sidewall 102 of rib 86'' and supported on spring
strip 98 attached to sidewall 102 by rivet 97. Thus as rib 86' is
lowered in the direction indicated by arrow 110 after being brought
into general alignment with rib 86'' the end of plug 89 guides and
engages the open end of rib 86''. As rib 86' descends, the taper of
plug 89 displaces pin 99 and bends and tensions spring strip 98 as
shown in FIG. 7B. With continued motion of rib 86' surface 92' of
rib 86' is brought into contact with surface 94' of rib 86'' and
recess 96 aligns with pin 99, which under the urging of tensioned
spring strip 98 is displaced into recess 96 as indicated in FIG.
7C. When configured as shown in FIG. 7C, layers 100 and 200 (FIG.
5) are fully restrained.
[0047] It will be appreciated that the specific locking mechanisms
and devices described above are intended to be illustrative and not
limiting and that other designs and configurations may be employed
without departing from the scope of the invention.
[0048] Returning to FIG. 5 it will be noted that layers 100 and 200
share a common wax sprue pattern 80'. Additional layers 100 (FIG.
4) may be incorporated and it is anticipated that a casting fixture
may include up to four layers. Additional layers would continue to
share a common wax sprue pattern developed by extension of sprue
pattern 80' at end 130 or end 132. Such a configuration would,
based on the configuration shown as 100 in FIG. 4 enable up to 24
rotors to be cast in a single pour. During casting and
solidification the casting fixture will be rotated about axis 81
coincident with the centerline of wax sprue pattern 80'. A suitable
direction of rotation is indicated by arrow 120, but rotation
opposite that shown by arrow 120 would also be effective.
[0049] The casting fixture is then used to create an investment, a
ceramic mold suitable for containing molten metal. Typically the
investment is produced by a series of sequential steps. First the
casting fixture is dipped into a slurry of fine refractory material
which will deposit as a thin layer on the fixture surfaces and then
letting any excess drain off, so that a uniform surface is
produced. The slurry may incorporate a variety of ceramics in
varying proportions ranging in size from about 45 to 75 micrometers
(200-325 mesh) and suitable to enable any fine details of the
finished casting to be accurately reproduced. Next, the casting
fixture is stuccoed, or overcoated with coarser ceramic particles,
including mullite, ranging in size from about 300 to 1000
micrometers (18-50 mesh), by dipping it into a fluidized bed,
placing it in a rain sander, or by applying by hand. Finally, the
coating is allowed to harden. These steps may be repeated to build
up the ceramic coating to the desired thickness, which is usually 5
to 15 mm (0.2 to 0.6 in).
[0050] Common refractory materials are used to create the
investments. These include: silica, zirconia, various aluminium
silicates, and alumina. The silica may be quartz or fused silica.
Aluminium silicates, mixture of alumina and silica, typically have
an alumina content ranging from 42 to 72% and include mullite at
72% alumina. Particularly during the initial slurry-based coat the
choice of refractory will be informed by the need to suppress
reaction between refractory and molten metal and may promote the
use of zirconia-based ceramics. The binders used to hold the
refractory material in place include: ethyl silicate (alcohol-based
and chemically set), colloidal silica or silica sol, set by drying,
sodium silicate, and a hybrid of these controlled for pH and
viscosity. Alcohol-based binders may be preferred in practice of
this invention to minimize corrosion of the ferrous lamination
materials. Where aqueous binders are used the laminations may be
protected by a thin barrier coating, for example of shellac,
applied by spraying or by dipping in a dilute solution with a
fast-evaporating and non-corrosive solvent.
[0051] Once the refractory has been applied in required thickness
and dried, the entire structure of FIG. 5 is enclosed in a
substantially-continuous layer of ceramic with all locations into
which molten metal is to cast being occupied with wax.
[0052] The wax is initially removed by gently heating the casting
fixture, for example in a steam autoclave, so that the wax will
melt and run out for collection and recycling. The casting fixture
is then `burned out`, that is heated to a temperature of about
1800-2200.degree. F. in an oxidizing atmosphere to combust and
remove all remnant wax and render the investment suitable for
receipt of the molten metal.
[0053] A fragmentary view of such an investment 100' is shown in
FIG. 8, which focuses on only a portion of the pattern shown in
FIG. 4, with the pattern shown in ghost for reference. Supporting
structure elements 82, 84 and 86, though not shown for clarity,
remain to provide support. The investment is covered by a
continuous layer of ceramic material 120. Removal of the wax has
created a sprue 80'' with a centerline 81 and runners 52'', shown
generally in ghost and in cut-away at location `A`. Thus molten
metal entering the sprue 80'', may be transported to
ceramic-encased lamination stack 11'' through runners 52''. Absent
the wax, ceramic-encased lamination stack will contain cavities
114'', 116'' and 117'' (commonly present in all of the
ceramic-encased lamination stacks but shown and identified only at
cutaway section `B`) suitable for transporting and accepting the
molten metal to form the conductor bars 14' and end rings 16', 17'
shown in FIG. 2.
[0054] The `burn out` step is also effective in preheating the
investment and thereby reducing the temperature difference between
the molten metal and investment during the casting process. The
preheated investment will be effective in increasing the fluidity
of the cast metal and act to prevent or minimize opportunity for
misruns during the casting process. The investment is then inserted
and positioned in a chamber or container which is agitated or
vibrated while sand of prescribed composition, typically mullite
although silica may also be used, and of minimal moisture content
with a distribution of particle sizes ranging from 150 to 840
micrometers (100-20 mesh) is added at a controlled rate. This
procedure will compact the sand around the investment, providing
support and rendering it capable of sustaining the, possibly at
least partially unbalanced, centrifugal forces generated during
casting. The assemblage of the container and its sand-supported
investment comprise the mold.
[0055] Because the lamination stacks comprising the rotors are
ferrous, they may function as chills during the casting process,
efficiently extracting heat from the inflowing molten metal,
lowering its temperature and causing it to freeze before the mold
is filled and producing misruns. To forestall this it is generally
desirable to at least preheat the investment, including the
lamination stacks, to a temperature at least close to the melting
point of the casting alloy. The preheating which occurs on burnout
may be adequate if sand fill, mold preparation and pouring occur
promptly, before the investment loses appreciable heat to the
poorly heat-conducting sand. However, although less preferred,
additional heat may be provided, for example by heating the mold in
an oven, prior to pouring if necessary.
[0056] The mold with its preheated investment is then rotated about
axis 81 (see FIGS. 5 and 8) with a rotational speed of between 1
and 300 rpm and the molten metal is introduced to the mold. For
example the molten metal may be top fed, by being poured into a
pouring cup, not shown, attached directly to end 130' of sprue 80''
or, more preferably with the addition of further melt distribution
channels (not shown), bottom fed, so that the molten metal enters
the mold at end 132' of sprue 80''. The molten metal will typically
be high purity electrical grade copper or aluminum to assure
minimal electrical resistance in the finished casting but it may be
preferred to use aluminum or copper alloys which may impart
additional strength. The use of such higher strength alloys will be
more preferred in higher performance motors which will subject the
rotor shorted structure to higher operating loads. The melt will be
maintained at some temperature greater than its melting
temperature, the excess being superheat, with the degree of
superheat and the investment temperature being cooperatively
selected to assure mold filling. The rotation imparted to the mold
will induce centrifugal forces directed outward to the periphery of
the investment and will promote radial flow outward along the
conductor bars where the individual metal flows will combine to
form the end ring.
[0057] Although the gating geometry is depicted as comprising a
common sprue and a single runner in FIG. 8, those skilled in the
art will recognize that alternative or supplementary gating or
venting may be beneficial in achieving consistent mold fill.
Similarly it will be appreciated that the casting process may be
conducted under at least partial vacuum and that the rotor forms
shown with the conductor bars aligned with the direction of the
applied centrifugal force may be inclined or otherwise oriented in
an alternative manner without departing from the gist of the
invention.
[0058] After the mold is filled, rotation is continued until
solidification is substantially complete. After solidification
concludes, the sand will be discharged from the mold, the
investment broken open and the gating removed to recover the rotor
with its cast shorted structure in conventional fashion.
[0059] The practice of the invention has been illustrated with some
exemplary designs and configurations which are not intended to
limit the scope of the invention.
* * * * *